Although a dir="oe" pin is generally equivalent to dir="io" pin with
the i* signal(s) disconnected, they are not equivalent, because some
pins may not be able to support input buffers at all, either because
there are no input buffers, or because the input buffers are consumed
by some other resource.
E.g. this can happen on iCE40 when the input buffer is consumed by
a PLL.
This is necessary because on some platforms, like iCE40, extras
become parameters on an IO primitive, since the constraint file
format is not expressive enough for all of them.
